1. Field of the Invention
The present invention generally relates to a fuel injection control system for controlling twin injectors.
2. Description of the Related Art
Conventionally, there is known an engine including so-called twin injectors (see e.g., Japanese Laid-Open Patent Application Publication No. 2006-132371). The twin injectors include a first injector (primary injector, or downstream injector), and a second injector (secondary injector, or upstream injector) positioned upstream of the first injector in an air flow direction.
The engine including the twin injectors can increase an amount of fuel which can be injected. Since the second injector is positioned more distant from a combustion chamber than the first injector, the fuel injected from the second injector is atomized more easily than the fuel injected from the first injector. When the atomization of the fuel progresses, air is cooled by vaporization and thereby air density increases. For these reasons, it is generally considered that the engine including the twin injectors can increase engine driving power. However, if the air supplied to the combustion chamber is insufficient in amount, the fuel injected from the second injector tends to stagnate in an air-intake passage, which makes it difficult to increase the engine driving power effectively.
To solve this problem, in the engine disclosed in the above publication, a conventional controller controls a setting of a fuel injection ratio of the first injector and a fuel injection ratio of the second injector, to be precise, a ratio of the amount of the fuel injected from the first injector with respect to a total fuel injection amount (a sum of the amount of the fuel injected from the first injector and the amount of the fuel injected from the second injector) and a ratio of the amount of the fuel injected from the second injector with respect to the total fuel injection amount (a sum of the fuel injection ratio of the first injector and the fuel injection ratio of the second injector is 1 (100%)). In this control, when the air amount is less or the engine speed is lower than a predetermined engine speed, the fuel injection ratio of the second injector is smaller than the fuel injection ratio of the first injector. When the air amount exceeds a predetermined amount, the fuel injection ratio of the second injector is greater than the fuel injection ratio of the first injector and is a constant value.
In this case, if the air amount and the engine speed increase rapidly in response to a driver's acceleration request, the fuel injection ratio of the second injector increases rapidly. The second injector is positioned away from the combustion chamber. If the fuel injection ratio of the second injector increases rapidly, the fuel injected from the second injector may reach the combustion chamber at a retarded timing.
On the other hand, since the first injector is positioned closer to the combustion chamber, the fuel injected from the first injector may reach the combustion chamber promptly. However, the fuel injection ratio of the first injector decreases rapidly according to the rapid increase in the fuel injection ratio of the second injector. Because of this, it is presumed that, immediately after the rapid increase in the fuel injection ratio of the second injector, the amount of the fuel actually supplied to the combustion chamber is undesirably less than the amount of the fuel supplied to the combustion chamber immediately before the rapid increase in the fuel injection ratio of the second injector.
In accordance with the conventional control, there is a chance that the amount of the fuel supplied actually to the combustion chamber will become much less than a desired amount for some time. This may result in a low output response during an acceleration state in which high engine driving power is necessary. Under these circumstances, an aim of the use of the twin injectors is not fulfilled.